The aim of the proposed research is to determine how an important form of DNA damage and a major substrate for human gene rearrangement-the DNA double strand break-is metabolized in human cells and cell extracts, and how DNA double strand break metabolism may be altered in Werner syndrome cells. Werner syndrome is an uncommon, autosomal recessive human disease associated with the premature appearance of features of normal aging in young adults, chromosomal instability and an increased risk of malignancy. Werner syndrome cells display a spontaneous mutator phenotype that reflects, at the molecular level, an increase in the rate of nonhomologous recombination-mediated deletions. Thus the Werner syndrome mutation provides an unusual opportunity to investigate an important human gene rearrangement pathway that involves DNA double strand break generation and rejoining, and to study the link between somatic mutations, gene rearrangement and the pathogenesis of clinically important, age-dependent disease processes such as cancer. Results of the proposed research will provide new and detailed molecular information on the DNA double strand break metabolism in human chromosomal genes, and will test the hypothesis that DNA double strand break joining is altered in Werner syndrome cells. The successful identification and characterization of a double strand break joining defect in Werner syndrome cells would have the additional benefit of providing useful biochemical and genetic assays for candidate Werner syndrome gene product identification and analysis.